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Magnetic-field-enhanced reactive synthesis of MnBi from Mn nanoparticles
Si, P.Z.,Yang, Y.,Yao, L.L.,Qian, H.D.,Ge, H.L.,Park, J.,Chung, K.C.,Choi, C.J. Elsevier 2019 Journal of magnetism and magnetic materials Vol.476 No.-
<P><B>Abstract</B></P> <P>The isotropic nanoparticles (NPs) and anisotropic bulk samples of MnBi were prepared from Mn NPs by using zero-field and field-assisted reactive sintering, respectively. The MnBi NPs are irregular in shape with size <I>d</I> < 600 nm, which is slightly larger than the precursor Mn NPs. The fraction of MnBi in the zero-field sintered NPs is higher than 68.6 wt%. The large surface area and small size of the Mn NPs enhanced the formation of ferromagnetic phase during sintering and the magnetic performance of the MnBi NPs, which show a coercivity (<I>H<SUB>c</SUB> </I>) up to 0.8 T and a remanent magnetization (<I>M<SUB>r</SUB> </I>) up to 44.5 Am<SUP>2</SUP>/kg at room temperature. The magnetic fields enhanced the formation of MnBi phase significantly and oriented the <I>c</I>-axis of the MnBi grains grown along the external fields during reactive sintering. A fraction of 63 wt% MnBi was formed in field-sintering within 2 h, whereas only 58.5 wt% MnBi was formed in zero-field-sintering for 48 h. The room-temperature <I>H<SUB>c</SUB> </I> and <I>M<SUB>r</SUB> </I> of the field-sintered bulk anisotropic MnBi reached up to 0.12–0.17 T and 37 Am<SUP>2</SUP>/kg, respectively.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Isotropic MnBi nanoparticles were prepared by reactive sintering of Mn nanoparticles and Bi. </LI> <LI> Anisotropic bulk MnBi was prepared by reactive sintering under a magnetic field of 5 T. </LI> <LI> Magnetic fields enhance the formation of MnBi and orient the <I>c</I>-axis of MnBi along the fields. </LI> </UL> </P>
Wang, X.L.,Si, P.Z.,Ge, H.L.,Shinde, K.P.,Chung, K.C.,Choi, C.J. Elsevier 2017 SOLID STATE SCIENCES Vol.67 No.-
<P><B>Abstract</B></P> <P>Ultra-high purity CrO<SUB>2</SUB> was prepared by decomposing CrO<SUB>3</SUB> in O<SUB>2</SUB> with gas pressures up to 40 MPa, which were maintained throughout the decomposition process of CrO<SUB>3</SUB> to prevent the formation of any other phases of chromium oxides. Our method is different from the traditional methods that start from or under ambient pressures. The high oxygen pressure makes the meta-stable CrO<SUB>2</SUB> stable from the initial stage of preparation. As a result, the purity of the as-prepared CrO<SUB>2</SUB> is improved, and this has been further proved by the highest magnetization of the samples. The as-prepared CrO<SUB>2</SUB> particles show very large grains with flat surfaces, octagonal cross-section, and straight edges, owing to the high mobility of Cr ions in CrO<SUB>2</SUB> at temperatures above its melting point. The lattice parameters of CrO<SUB>2</SUB> are <I>a</I> = 4.4176 Å and <I>c</I> = 2.9144 Å. The maximum value of the magnetic entropy change of the high purity CrO<SUB>2</SUB> particles is ∼2.83 J/kg·K for an applied field of 1.5 T. The preparation of pure CrO<SUB>2</SUB> is important for studying its intrinsic properties and for applications in spintronic devices.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Ultra-high purity CrO<SUB>2</SUB> was prepared. </LI> <LI> High O<SUB>2</SUB>-gas pressure was employed. </LI> <LI> The CrO<SUB>2</SUB> rods show straight edges and high magnetization. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Indirect-direct band gap transition through electric tuning in bilayer MoS2.
Zhang, Z Y,Si, M S,Wang, Y H,Gao, X P,Sung, Dongchul,Hong, Suklyun,He, Junjie American Institute of Physics 2014 The Journal of chemical physics Vol.140 No.17
<P>We investigate the electronic properties of bilayer MoS2 exposed to an external electric field by using first-principles calculations. It is found that a larger interlayer distance, referring to that by standard density functional theory (DFT) with respect to that by DFT with empirical dispersion corrections, makes indirect-direct band gap transition possible by electric control. We show that external electric field effectively manipulates the valence band contrast between the K- and γ-valleys by forming built-in electric dipole fields, which realizes an indirect-direct transition before a semiconductor-metal transition happens. Our results provide a novel efficient access to tune the electronic properties of two-dimensional layered materials.</P>
P. Z. Si,X. L. Wang,X. F. Xiao,H. J. Chen,X. Y. Liu,L. Jiang,J. J. Liu,Z. W. Jiao,H. L. Ge 한국자기학회 2015 Journal of Magnetics Vol.20 No.3
Cr₂O₃ nanoparticles were prepared via one-step reactive laser ablation of Cr in oxygen. The metastable CrO₂ phase was obtained through the subsequent oxidation of Cr₂O₃ nanoparticles under O₂ with gas pressures of up to 40 MPa. The as-prepared Cr₂O₃ nanoparticles are spherical or rectangular in shape with sizes ranging from 20 nm to 50 nm. High oxygen pressure annealing is effective in producing meta-stable CrO₂ from as-dried Cr₂O₃ nanoparticles, and the Cr₂O₃ nanoparticles exhibit a weak ferromagnetic behavior with an exchange bias of up to 11 mT that can be ascribed to the interfacial exchange coupling between uncompensated surface spins and the antiferromagnetic core. The Cr₂O₃/CrO₂ nanoparticles exhibit an enhanced saturation magnetization and a reduced exchange bias with an increasing faction of CrO₂ due to the elimination of uncompensated surface spins over the Cr₂O₃ nanoparticles when exposed to a high pressure of O₂ and/or possible phase segregation that results in a smaller grain size for both Cr₂O₃ and CrO₂.
Weak Ferromagnetism and Exchange Bias in Antiferromagnetic Cobalt Oxide Nanoparticles
X. L. Wang,H. L. Ge,Q. L. Ye,P. Z. Si,H. J. Chen Korean Magnetics Society 2018 Journal of Magnetics Vol.23 No.4
The traditional arc discharge process for producing metallic nanoparticles was modified to be applicable for the direct preparation of oxide nanoparticles by using both conductive and insulative precursors. The cobalt oxide nanoparticles were synthesized by feeding Co and Co₂O₃ into the air plasma. The as-prepared nanoparticles are spherical in shape with size in the range of 10 to 40 nm. The phases of CoO and Co₃O₄ were detected in the samples prepared from the precursors of Co and Co₂O₃, respectively. Weak ferromagnetism and an exchange bias up to 17.1 mT were observed in these antiferromagnetic nanoparticles, owing to the presence of a large fraction of the uncompensated surface spins and possibly the local disordered regions.